Quadcopters generally use two pairs of identical fixed pitched propellers; two clockwise (CW) and two counterclockwise (CCW). These use independent variation of the speed of each rotor to achieve control. By changing the speed of each rotor it is possible to specifically generate a desired total thrust; to locate for the centre of thrust both laterally and longitudinally; and to create a desired total torque, or turning force.[2]

Quadcopters differ from conventional helicopters, which use rotors that are able to vary the pitch of their blades dynamically as they move around the rotor hub. In the early days of flight, quadcopters (then referred to either as 'quadrotors' or simply as 'helicopters') were seen as possible solutions to some of the persistent problems in vertical flight. Torque-induced control issues (as well as efficiency issues originating from the tail rotor, which generates no useful lift) can be eliminated by counter-rotation, and the relatively short blades are much easier to construct. A number of manned designs appeared in the 1920s and 1930s. These vehicles were among the first successful heavier-than-air vertical take off and landing (VTOL) vehicles.[3] However, early prototypes suffered from poor performance,[3] and latter prototypes required too much pilot work load, due to poor stability augmentation[4] and limited control authority.

In the late 2000s, advances in electronics allowed the production of cheap lightweight flight controllers, accelerometers (IMU), global positioning system and cameras. This resulted in the quadcopter configuration becoming popular for small unmanned aerial vehicles. With their small size and maneuverability, these quadcopters can be flown indoors as well as outdoors.[1][5]

At a small size, quadcopters are cheaper and more durable than conventional helicopters due to their mechanical simplicity.[6] Their smaller blades are also advantageous because they possess less kinetic energy, reducing their ability to cause damage. For small-scale quadcopters, this makes the vehicles safer for close interaction. It is also possible to fit quadcopters with guards that enclose the rotors, further reducing the potential for damage.[7] However, as size increases, fixed propeller quadcopters develop disadvantages relative to conventional helicopters. Increasing blade size increases their momentum. This means that changes in blade speed take longer, which negatively impacts control. Helicopters do not experience this problem as increasing the size of the rotor disk does not significantly impact the ability to control blade pitch.

Due to their ease of construction and control, quadcopter aircraft are frequently used as amateur model aircraft projects.[8][9]

A four-rotor helicopter designed by Louis Breguet. This was the first rotary wing aircraft to lift itself off the ground, although only in tethered flight at an altitude of a few feet. In 1908 it was reported as having flown 'several times', although details are sparse.[10]

Oehmichen No.2 (1920)

Etienne Oehmichen experimented with rotorcraft designs in the 1920s. Among the six designs he tried, his helicopter No.2 had four rotors and eight propellers, all driven by a single engine. The Oehmichen No.2 used a steel-tube frame, with two-bladed rotors at the ends of the four arms. The angle of these blades could be varied by warping. Five of the propellers, spinning in the horizontal plane, stabilized the machine laterally. Another propeller was mounted at the nose for steering. The remaining pair of propellers functioned as its forward propulsion. The aircraft exhibited a considerable degree of stability and increase in control-accuracy for its time, and made over a thousand test flights during the middle 1920s. By 1923 it was able to remain airborne for several minutes at a time, and on April 14, 1924 it established the first-ever FAI distance record for helicopters of 360 m (390 yd). It demonstrated the ability to complete a circular course[11] and later, it completed the first 1 kilometre (0.62 mi) closed-circuit flight by a rotorcraft.

Dr. George de Bothezat and Ivan Jerome developed this aircraft, with six-bladed rotors at the end of an X-shaped structure. Two small propellers with variable pitch were used for thrust and yaw control. The vehicle used collective pitch control. Built by the US Air Service, it made its first flight in October 1922. About 100 flights were made by the end of 1923. The highest it ever reached was about 5 m (16 ft 5 in). Although demonstrating feasibility, it was underpowered, unresponsive, mechanically complex and susceptible to reliability problems. Pilot workload was too high during hover to attempt lateral motion.

This unique helicopter was intended to be the prototype for a line of much larger civil and military quadrotor helicopters. The design featured two engines driving four rotors through a system of v belts. No tail rotor was needed and control was obtained by varying the thrust between rotors.[12] Flown successfully many times in the mid-1950s, this helicopter proved the quadrotor design and it was also the first four-rotor helicopter to demonstrate successful forward flight. Due to a lack of orders for commercial or military versions however, the project was terminated. Convertawings proposed a Model E that would have a maximum weight of 42,000 lb (19 t) with a payload of 10,900 lb (4.9 t) over 300 miles and at up to 173 mph (278 km/h). The Hanson Elastic Articulated (EA) bearingless rotor grew out of work done in the early 1960s at Lockheed California by Thomas F. Hanson, who had previously worked at Convertawings on the quadrotor's rotor design and control system.[13][14]

The Curtiss-Wright VZ-7 was a VTOL aircraft designed by the Curtiss-Wright company for the US Army. The VZ-7 was controlled by changing the thrust of each of the four propellers. Two VZ-7 prototypes were created and both performed adequately during the initial flight test program, and both did reasonably well when evaluated by the Army. The VZ-7 was capable of hovering and forward flight and proved relatively stable and easy to operate. The aircraft was retired in 1960 due to not meeting military standards.[15]

In the last few decades, small-scale unmanned aerial vehicles have been used for many applications. The need for aircraft with greater maneuverability and hovering ability has led to a rise in quadcopter research. The four-rotor design allows quadcopters to be relatively simple in design yet highly reliable and maneuverable. Research is continuing to increase the abilities of quadcopters by making advances in multi-craft communication, environment exploration, and maneuverability. If these developing qualities can be combined, quadcopters would be capable of advanced autonomous missions that are currently not possible with other vehicles.[16]

Some current programs include:

The Bell Boeing Quad TiltRotor concept takes the fixed quadcopter concept further by combining it with the tilt rotor concept for a proposed C-130 sized military transport.

Quadcopters are a useful tool for university researchers to test and evaluate new ideas in a number of different fields, including flight control theory, navigation, real time systems, and robotics. In recent years many universities have shown quadcopters performing increasingly complex aerial manoeuvres. Swarms of quadcopters can hover in mid-air,[32][33][34][35] fly in formations,[36][37][38][39][40] and autonomously perform complex flying routines such as flips, darting through hula hoops and organising themselves to fly through windows as a group.[41]

There are numerous advantages to using quadcopters as versatile test platforms. They are relatively cheap, available in a variety of sizes and their simple mechanical design means that they can be built and maintained by amateurs. Due to the multi-disciplinary nature of operating a quadcopter, academics from a number of fields need to work together in order to make significant improvements to the way quadcopters perform. Quadcopter projects are typically collaborations between computer science, electrical engineering and mechanical engineering specialists.[42]

Quadcopter unmanned aerial vehicles are used for surveillance and reconnaissance by military and law enforcement agencies, as well as search and rescue missions in urban environments.[43] One such example is the Aeryon Scout, created by Canadian company Aeryon Labs,[44] which is a small UAV that can quietly hover in place and use a camera to observe people and objects on the ground. The company claims that the quadrotor played a key role in a drug raid in Central America by providing visual surveillance of a drug trafficker's compound deep in the jungle (Aeryon has declined to name the exact country and provide other specific details).[45]

The weaponization of drones is now a huge concern to the wider security industry, as is their use to disrupt airspace, as seen at Gatwick Airport in the UK in December 2018,[46] prompting the United Kingdom’s Civil Aviation Authority, via Parliament to make expedient changes to UK drone laws.[47]

After a recreational quadcopter (or "drone") crashed on the White House lawn early in the morning of January 26, 2015,[48] the Secret Service began a series of test flights of such equipment in order to fashion a security protocol against hostile quadcopters.[49]

During the Battle of Mosul it was reported that commercially available quadcopters and drones were being used by Islamic State of Iraq and the Levant (ISIL) as surveillance and weapons delivery platforms using improvised cradles to drop grenades and other explosives.[50] The ISIL drone facility became a target of Royal Air Force strike aircraft.[51]

The largest use of quadcopters in the USA has been in the field of aerial imagery. Quadcopter UAVs are suitable for this job because of their autonomous nature and huge cost savings.[16]
Drones have also been used for light-painting photography.[52][53][54]

In 2014 The Guardian reported that major media outlets have started to put serious effort into exploring the use of drones for reporting and verifying news on events that include floods, protests and wars.[55]

Some media outlets and newspapers are using drones to capture photography of celebrities.[56]

In December 2013, the Deutsche Post gathered international media attention with the project Parcelcopter, in which the company tested the shipment of medical products by drone delivery. Using a Microdrones md4-1000 quadrocopter, packages were flown from a pharmacy across the Rhine River. It was the first civilian package delivery via drones.[57][58]

Quadcopters are being used for a wide variety of humanitarian applications from disaster relief to animal conservation. During Hurricane Harvey, many drone pilots with quadcopters at their disposal descended on the city of Houston, Texas to provide support to first responders.[59]

Quadcopters have also been used in various art projects including but not limited to drone photography. At least one drone has demonstrated the feasibility of painting graffiti on a wall with spray paint.[60] They may be used in performance art with new degrees of positional control that allows for new uses of puppets, characters, lights and cameras.[61] They have also been used in light shows[62] including most prominently in the 5 February 2017 Super Bowl LI halftime show in which Lady Gaga, in a pre-recorded segment, was accompanied by a swarm of 300 LED-equipped Intel "Shooting Star" drones forming an American flag in the sky.[63][64][65][66]

Quadcopters are used all over the world for racing (also known as "drone racing") and freestyle events.[67] Racing and freestyle quadcopters are built for speed and agility. Racing and freestyle drones tend to be relatively small in size, with 250 mm between the propeller shafts and/or 5-6 inch props being the usually upper end of the size scale.[68]

There are at least three international drone racing organisations/promotions including the Drone Racing League[69] ,Multi GP.[70] And the Drone Championship League[71]

In the United States, the legality of the use of remotely controlled aircraft for commercial purposes has been the source of legal issues. Raphael Pirker, a professional photographer, was fined by the FAA in 2012 for "endangering people on the ground" (a regulatory infraction) after he used a Zephyr fixed-wing drone—a "five-pound Styrofoam model airplane"—to take aerial photos of the University of Virginia's campus in 2011.[72][73] In March 2014, a federal administrative law judge ruled in Pirker's favor, determining that his drone was a "model aircraft" and thus not subject to FAA regulations on other types of aircraft.[72] The FAA appealed to the National Transportation Safety Board; the NTSB appointed a new administrative law judge, who overturned the earlier finding and ruled that under the FAA's enabling act, the FAA had jurisdiction to regulate "any contrivance invented, used or designed to navigate, or fly in, the air," irrespective of whether it was unmanned or manned.[72] Pirker was fined $10,000, but in January 2015 settled the matter with the FAA, agreeing to pay a $1,100 fine without admitting guilt.[72] According to a report in Aviation Week, the matter "became a cause célèbre among the model aircraft and recreational and commercial small drone communities."[72]

In December 2014, the FAA released a video detailing many best practices for new drone pilots, including advisories such as keeping their machines below 400 feet and always within visual sight.[74]

As of March 2015, the United States created an interim policy for the legal use of unmanned aerial vehicles for commercial use where each operator can apply for an exemption filed under Section 333 with the FAA. As of August 2015 the FAA had granted over 1300 petitions to different use cases and industries.[75] Furthermore, FAA has started discussions in November 2015 to require all hobbyists to also register personal drones to FAA website.[76]

In December 2014, FAA started the registration process for all model aircraft weighing more than 0.55 pounds (≈ 250 grams) and less than 55 pounds (≈ 25 kilograms), including payloads such as all on-board cameras and other accessories. FAA's decision on the matter caused resistance from the hobbyist community at the time. In October 2015 the FAA issued rules legally requiring registration, starting January 2016, but on May 19, 2017, the United States Court of Appeals for the Washington D.C. circuit ruled that this violated the 2012 FAA Modernization and Reform Act, which states that the FAA “may not promulgate any rule or regulation regarding a model aircraft.”[77] The FAA continues to allow registration on a voluntary basis, as well as requiring it for commercial use, but states that it is not required if "flying under the Special Rule for Model Aircraft" (recreational hobby use).[78]

In addition to the registration requirement, FAA has also released various operational requirements as follows:[79]

Flight only below 400 feet above ground.

The operator must maintain visual contact with the aircraft at all times.

Not fly near manned aircraft, especially near airports.

Not fly over groups of people, stadiums or sporting events.

Not fly near emergency response efforts, such as accident sites or forest fires.

On October 5, 2018, the FAA Reauthorization Act of 2018 passed congress and was sent for the president's signature. The bipartisan bill split the hobbyist community, with the Commercial Drone Alliance in favor of the new law, while the Academy of Model Aeronautics urged its members to oppose the bill.[80]

The major change in the new law is the repeal of Section 336. All sUAS owners, including quadcopters, will be required to register their aircraft and pass a general aeronautical knowledge test. In June 2019, the FAA announced new rules for recreational and non-commercial fliers. Recreational pilots would no longer be allowed to fly in controlled airspace by contacting air traffic control towers. This rule will be in effect until the LAANC system for hobbyists is rolled out in the summer of 2019.[81]

Each rotor produces both a thrust and torque about its center of rotation, as well as a drag force opposite to the vehicle's direction of flight. If all rotors are spinning at the same angular velocity, with rotors one and three rotating clockwise and rotors two and four counterclockwise, the net aerodynamic torque, and hence the angular acceleration about the yaw axis, is exactly zero, which means there is no need for a tail rotor as on conventional helicopters. Yaw is induced by mismatching the balance in aerodynamic torques (i.e., by offsetting the cumulative thrust commands between the counter-rotating blade pairs).[82][83]

Schematic of reaction torques on each motor of a quadcopter aircraft, due to spinning rotors. Rotors 1 and 3 spin in one direction, while rotors 2 and 4 spin in the opposite direction, yielding opposing torques for control.

The red propellers are moving clockwise while the blue propellers are moving counterclockwise.

A quadrotor hovers or adjusts its altitude by applying equal thrust to all four rotors.

A quadrotor adjusts its yaw by applying more thrust to rotors rotating in one direction.

A quadrotor adjusts its pitch or roll by applying more thrust to one rotor and less thrust to its diametrically opposite rotor.

In order to allow more power and stability at reduced weight, a quadcopter, like any other multirotor can employ a coaxial rotor configuration. In this case, each arm has two motors running in opposite directions (one facing up and one facing down).[84]

The main mechanical components needed for construction are the frame, propellers (either fixed-pitch or variable-pitch), and the electric motors. For best performance and simplest control algorithms, the motors and propellers should be placed equidistant.[86] Recently, carbon fiber composites have become popular due to their light weight and structural stiffness.

The electrical components needed to construct a working quadcopter are similar to those needed for a modern RC helicopter. They are the electronic speed control module, on-board computer or controller board, and battery (or fuel cell[87]). Typically, a hand-held transmitter is also used to allow for human input.[88]

Quadcopters and other multicopters often can fly autonomously. Many modern flight controllers use software that allows the user to mark "way-points" on a map, to which the quadcopter will fly and perform tasks, such as landing or gaining altitude.[89] The PX4 autopilot system, an open-source software/hardware combination in development since 2009, has since been adopted by both hobbyists and drone manufacturing companies alike to give their quadcopter projects flight-control capabilities. Other flight applications include crowd control between several quadcopters where visual data from the device is used to predict where the crowd will move next and in turn direct the quadcopter to the next corresponding waypoint.[90]

Electric quadcopters have shorter endurance when compared to similarly sized electric helicopters because of the smaller "disk area" of the rotor blades. The longest flight time achieved by a battery-powered quadcopter was 2 hours, 31 minutes and 30 seconds. The record was set by Ferdinand Kickinger of Germany in 2016.[91] In setting the record, Kickinger used low-discharge rate, high capacity Lithium Ion batteries and stripped the airframe of non-essential weight to reduce power draw and extend endurance. However, the lack of features and the use of a low discharge battery make the vehicle unusable as a commercial product.[92]

Alternative power sources like hydrogen fuel cells and hybrid gas-electric generators have been used to dramatically extend the endurance of quadcopters and multirotors in general because of the increased energy density of both hydrogen and gasoline, respectively.[93]